I was wondering about the difference between "ab nun", "ab sofort" and "ab jetzt".
I think that most of the time, speaking, I just heard "ab jetzt".
Are the other two expression the same? Or is there a difference in usage? In case what is it?
I never heard »ab nun«. I think it is correct, but rarely used. And since I never heard it, I am not able to tell what it should mean exactly. (I live in Austria, and here the word »nun« is much rarer used then in Germany, so when I say I did not hear this phrase, then this has not much to say.)
»Ab sofort« means »from this second on«.
Imagine a moderator of a life TV show who says at the end of a telephone voting:
Die Telefon-Leitungen sind ab sofort wieder geschlossen.
The telephone lines are closed again from now on.
And it means that they was open until the moment when he said this sentence, so »ab sofort« has a very accurate meaning.
Now imagine an author of a book who says at an interview:
Mein neues Buch gibt es ab jetzt auch im Buchhandel zu kaufen.
My new book is available in book stores from now on.
But it doesn't mean that it was not available before he said this sentence. Most probably the book is available in 90 % of all stores since yesterday, and in the rest of the shops it will be available soon.
So in »ab jetzt« the word »jetzt« must be interpreted much less accurate then »sofort« in »ab sofort«.
On the other hand, the moderator could also have said
Die Telefon-Leitungen sind ab jetzt wieder geschlossen.
and the audience also would understand that they was open just before he said this sentence, but when he says »sofort«, it feels sharper and more precise.
Also the Author could have said:
Mein neues Buch gibt es ab sofort auch im Buchhandel zu kaufen.
And also hear everybody would understand that the books are not rolled out in this very second. So the context has a much stronger influence on the meaning then the words in the phrase.
When the author says »ab sofort«, then it sounds more like an invitation to buy this book.
Maybe the focus of »ab sofort« is more like »The Future is beginning exactly in this moment!« while »ab jetzt« has more the flavor of »The past ends now, in this days.«
»Jetzt« and »sofort« are beside the just discussed phrases not interchangeable. It's similar to the closest English translations: »Jetzt» is »now« and »sofort« is »immediately«, and there are not much usages of those words where you can replace one with the other.